U.S. patent number 7,232,790 [Application Number 10/332,845] was granted by the patent office on 2007-06-19 for activated carbon, method for production thereof and use thereof.
This patent grant is currently assigned to Showa Denko K.K.. Invention is credited to Yasuo Saito, Masako Tanaka.
United States Patent |
7,232,790 |
Tanaka , et al. |
June 19, 2007 |
Activated carbon, method for production thereof and use thereof
Abstract
A method for producing an activated carbon material, wherein the
method comprises a step of thermally treating coal-based pitch at
two temperature ranges of 400.degree. C. to 600.degree. C. and
600.degree. C. to 900.degree. C.; and a step of mixing the thus
obtained carbonaceous material with an alkali metal compound and
effecting activation thereof at 600.degree. C. to 900.degree. C.,
and an activated carbon material obtained by the method. When the
activated carbon material of the present invention is used a
polarizable electrode material of an electric double layer
capacitor, high capacitance per electrode is attained without
application of excessive voltage. By adding fibrous material to a
coal-based pitch during activation expansion of an alkali molten
liquid can be suppressed and productivity can be drastically
improved. Furthermore, employment of an fibrous carbon material
which is excellent in conductivity as a fibrous material, carbon
fiber is melt-bonded on the surface of the activated carbon
material, which enables production of a polarizable electrode
exhibiting excellent charge/discharge characteristics at high
current density.
Inventors: |
Tanaka; Masako (Kanagawa,
JP), Saito; Yasuo (Kanagawa, JP) |
Assignee: |
Showa Denko K.K. (Tokyo,
JP)
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Family
ID: |
28046062 |
Appl.
No.: |
10/332,845 |
Filed: |
September 9, 2002 |
PCT
Filed: |
September 09, 2002 |
PCT No.: |
PCT/JP02/09151 |
371(c)(1),(2),(4) Date: |
January 14, 2003 |
PCT
Pub. No.: |
WO03/024868 |
PCT
Pub. Date: |
March 27, 2003 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20030179537 A1 |
Sep 25, 2003 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60380858 |
May 17, 2002 |
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60318623 |
Sep 13, 2001 |
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Foreign Application Priority Data
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Sep 11, 2001 [JP] |
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2001-274375 |
Nov 22, 2001 [JP] |
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2001-357735 |
Apr 11, 2002 [JP] |
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2002-109638 |
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Current U.S.
Class: |
502/427 |
Current CPC
Class: |
H01G
9/155 (20130101); C01B 32/342 (20170801); B01J
20/20 (20130101); B01J 20/30 (20130101); Y02E
60/13 (20130101); Y10T 428/2918 (20150115); Y10T
428/298 (20150115) |
Current International
Class: |
C01B
31/12 (20060101) |
Field of
Search: |
;502/427,432 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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03-132009 |
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Jun 1991 |
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JP |
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03-237011 |
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Oct 1991 |
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JP |
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05-009812 |
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Jan 1993 |
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JP |
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06-267794 |
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Sep 1994 |
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JP |
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09-187648 |
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Jul 1997 |
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JP |
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11-317333 |
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Nov 1999 |
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JP |
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2000-138140 |
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May 2000 |
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JP |
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WO 00/12207 |
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Mar 2000 |
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WO |
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Primary Examiner: Hendrickson; Stuart
Attorney, Agent or Firm: Sughrue Mion, PLLC
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is based on the provisions of 35 U.S.C. Article
111(a) with claiming the benefit of filing dates of U.S.
provisional application Ser. No. 60/318,623 filed on Sep. 13, 2001
and No. 60/380,858 filed on May 17, 2002 under the provisions of 35
U.S.C. 111(b), pursuant to 35 U.S.C. Article 119(e)(1).
Claims
The invention claimed is:
1. A method for producing an activated carbon material, wherein the
method comprises a step of thermally treating coal-based pitch with
a two-stage heat treatment at two temperature ranges of 400.degree.
C. to 600.degree. C. and 600.degree. C. to 900.degree. C.; and a
step of mixing and heating the thus-treated coal-based pitch with
an alkali metal compound for the activation thereof, and wherein
the step of thermally treating coal-based pitch at two temperature
ranges is carried out in a vapor of an alkali metal.
2. The method for producing an activated carbon material as claimed
in claim 1, wherein the alkali metal compound is at least one
alkali hydroxide selected from the group consisting of sodium
hydroxide, potassium hydroxide, and cesium hydroxide.
3. The method for producing an activated carbon material as claimed
in claim 1, wherein the two temperature ranges are 450.degree. C.
to 600.degree. C. and 600.degree. C. to 900.degree. C.
4. The method for producing an activated carbon material as claimed
in claim 1, wherein the alkali metal compound is at least one
species selected from the group consisting of potassium, sodium,
and cesium compounds.
5. The method for producing an activated carbon material as claimed
in claim 1, wherein the step for the activation comprises adding a
fibrous material to the coal-based pitch.
6. The method for producing an activated carbon material as claimed
in claim 5, wherein the amount of the fibrous material is not less
than 0.05 mass % as a corresponding mass of the fibrous material
heated at 800.degree. C. in an inert atmosphere on the basis of the
coal-based pitch.
7. The method for producing an activated carbon material as claimed
in claim 5, wherein the outer diameter of each fiber filament of
the fibrous material is 1000 nm or less.
8. The method for producing an activated carbon material as claimed
in claim 5, wherein the fibrous material is a material capable of
maintaining its shape up to at least 300.degree. C.
9. The method for producing an activated carbon material as claimed
in claim 5, wherein the fibrous material is at least one species
selected from the group consisting of a fibrous carbon, carbonized
material of organic fiber, unmeltable fiber, beaten pulp and
cellulose fiber.
10. The method for producing an activated carbon material as
claimed in claim 9, wherein the fibrous carbon is at least one
species selected from the group consisting of a carbon nano tube,
whiskers, vapor grown carbon fiber, carbon fiber ribbon and coiled
carbon fiber.
11. The method for producing an activated carbon material as
claimed in claim 10, wherein each fiber filament of the vapor grown
carbon fiber contains a hollow space extending along its center
axis, and has an outer diameter of 2 to 500 nm and an aspect ratio
of 10 to 15,000.
12. The method for producing an activated carbon material as
claimed in claim 11, wherein the vapor grown carbon fiber is
branched carbon fiber.
13. A method for producing an activated carbon material, wherein
the method comprises adding an alkali metal compound as an
activating agent and a fibrous material to a carbonaceous raw
material to form a mixture and heating the mixture to activate the
carbonaceous raw material, wherein the fibrous material comprises
filaments having an outer diameter of 1000 nm or less or fibrils
having an outer diameter of 1000 nm or less.
14. The method for producing an activated carbon material as
claimed in claim 13, wherein the amount of the fibrous material is
not less than 0.05 mass % as a corresponding mass of the fibrous
material heated at 800.degree. C. in an inert atmosphere on the
basis of the carbonaceous raw material.
15. The method for producing an activated carbon material as
claimed in claim 13, wherein the outer diameter of each fiber
filament of the fibrous material is 1000 nm or less.
16. The method for producing an activated carbon material as
claimed in claim 13, wherein the fibrous material is a material
capable of maintaining its shape up to at least 300.degree. C.
17. The method for producing an activated carbon material as
claimed in claim 13, wherein the fibrous material is at least one
species selected from the group consisting of a fibrous carbon,
carbonized material of organic fiber, unmeltable fiber, beaten pulp
and cellulose fiber.
18. The method for producing an activated carbon material as
claimed in claim 17, wherein the fibrous carbon is at least one
species selected from the group consisting of a carbon nano tube,
whiskers, vapor grown carbon fiber, carbon fiber ribbon and coiled
carbon fiber.
19. The method for producing an activated carbon material as
claimed in claim 18, wherein each fiber filament of the vapor grown
carbon fiber contains a hollow space extending along its center
axis, and has an outer diameter of 2 to 500 nm and an aspect ratio
of 10 to 15,000.
20. The method for producing an activated carbon material as
claimed in claim 19, wherein the vapor grown carbon fiber is
branched carbon fiber, and each fiber filament of the branched
carbon fiber contains a hollow space extending throughout the
filament, including a branched portion thereof.
21. A method for producing an activated carbon material, wherein
the method comprises a step of thermally treating coal-based pitch
with a two-stage heat treatment at two temperature ranges of
400.degree. C. to 600.degree. C. and 600.degree. C. to 900.degree.
C.; and a step of mixing and heating the thus-treated coal-based
pitch with an alkali metal compound for the activation thereof,
wherein a softening point of the coal-based pitch is 100.degree. C.
or lower.
22. The method for producing an activated carbon material as
claimed in claim 21, wherein the temperature increase rate is 3 to
10.degree. C./hour in the thermally treating step in the range of
400.degree. C. to 600.degree. C.
23. The method for producing an activated carbon material as
claimed in claim 21, wherein the step for the activation comprises
adding a fibrous material to the coal-based pitch.
24. The method for producing an activated carbon material as
claimed in claim 23, wherein the amount of the fibrous material is
not less than 0.05 mass % as a corresponding mass of the fibrous
material heated at 800.degree. C. in an inert atmosphere on the
basis of the coal-based pitch.
25. The method for producing an activated carbon material as
claimed in claim 23, wherein the outer diameter of each fiber
filament of the fibrous material is 1000 nm or less.
26. The method for producing an activated carbon material as
claimed in claim 23, wherein the fibrous material is a material
capable of maintaining its shape up to at least 300.degree. C.
27. The method for producing an activated carbon material as
claimed in claim 23, wherein the fibrous material is at least one
species selected from the group consisting of a fibrous carbon,
carbonized material of organic fiber, unmeltable fiber, beaten pulp
and cellulose fiber.
28. The method for producing an activated carbon material as
claimed in claim 27, wherein the fibrous carbon is at least one
species selected from the group consisting of a carbon nano tube,
whiskers, vapor grown carbon fiber, carbon fiber ribbon and coiled
carbon fiber.
29. The method for producing an activated carbon material as
claimed in claim 28, wherein each fiber filament of the vapor grown
carbon fiber contains a hollow space extending along its center
axis, and has an outer diameter of 2 to 500 nm and an aspect ratio
of 10 to 15,000.
30. The method for producing an activated carbon material as
claimed in claim 29, wherein the vapor grown carbon fiber is
branched carbon fiber.
Description
TECHNICAL FIELD
The present invention relates to an activated carbon material which
can be employed in a variety of uses including treatment of tap
water or wastewater, and as catalyst carrier, gas occlusion
material, electrode material for an electric double layer capacitor
(also called "electric double layer condenser") and the like as
well as to a method for producing the activated carbon material.
The present invention also relates to a polarizable electrode
containing the activated carbon material and to an electric double
layer capacitor containing the electrode and exhibiting high
capacitance and high durability.
BACKGROUND ART
Carbon material, particularly activated carbon material, is
employed in a variety of fields; for example, it finds utility in
treatment of water, catalyst carrier, gas occlusion and electric
double layer capacitor electrodes. Among these, an electric double
layer capacitor exhibits, for example, the following
characteristics: rapid charging and discharging; resistance to
excessive charging and discharging; long service life (since it
does not undergo chemical reaction); a wide temperature range in
which the capacitor can be used; and environmentally friendly
nature (since it contains no heavy metal). Therefore,
conventionally, an electric double layer capacitor has been
employed in, for example, a memory backup power supply. In recent
years, electric double layer capacitors of high capacitance have
been developed rapidly, and such electric double layer capacitors
have been employed in high-performance energy devices. Furthermore,
an electric double layer capacitor has been envisaged to be
employed in a power storage system in combination with a solar
battery or a fuel cell, or to be employed for assisting a gasoline
engine of a hybrid car.
An electric double layer capacitor includes a pair of positive and
negative polarizable electrodes formed of, for example, activated
carbon, the electrodes facing each other with the intervention of a
separator in a solution containing electrolyte ions. When DC
voltage is applied to the electrodes, anions contained in the
solution migrate to the positively polarized electrode, and cations
contained in the solution migrate to the negatively polarized
electrode. Electric energy is obtained from an electric double
layer formed at the interface between the solution and each of the
electrodes.
Conventional electric double layer capacitors are excellent in
power density but poor in energy density. Therefore, in order to
realize employment of such an electric double layer capacitor in
energy devices, capacitance of the capacitor must be increased
further. In order to increase capacitance of an electric double
layer capacitor, an electrode material which enables formation of a
large number of electric double layers in an electrolytic solution
must be developed.
An electrode predominantly containing activated carbon material is
employed as a component constituting an electric double layer
capacitor. Such activated carbon material is required to exhibit,
as a key function, high capacitance per mass or per volume.
In view of the foregoing, use of an activated carbon material
having a large specific surface area has been contemplated as an
electrode material which enables formation of a large number of
electric double layers. When such an activated carbon material is
employed, capacitance per mass (F/g) increase, but capacitance per
volume (F/ml) fails to increase to an intended level, because of
lowering of electrode density.
In recent years, there has been proposed an approach to production
of an activated carbon material containing microcrystals similar to
those of graphite, along with employment of the thus-produced
activated carbon material as a raw material for forming a
polarizable electrode (Japanese Patent Application Laid-Open
(kokai) No. 11-317333). In view that an electric double layer
capacitor including a polarizable electrode formed from the
activated carbon material exhibits high capacitance, the activated
carbon material is considered an excellent electrode material.
However, the aforementioned activated carbon material is not
necessarily satisfactory, in that it involves some problems. That
is, since expansion of the activated carbon material occurs during
application of voltage, as described in the above publication, a
size-limiting structure must be provided for suppressing expansion
of the activated carbon material, and thus difficulty is
encountered in assembling a capacitor. In addition, application of
a voltage of as high as about 4 V is required in advance in order
to obtain sufficient capacitance of the capacitor. As a result,
decomposition of an electrolytic solution may occur.
In a typical method for producing activated carbon material, an
organic substance such as coconut shell, pitch, or phenol resin is
thermally decomposed to thereby yield a carbonized material, and
the carbide is activated.
Examples of activation methods include gas activation employing
steam or carbon dioxide gas, and chemical activation employing, for
example, potassium sulfide, zinc chloride, or an alkali hydroxide.
Particularly, activation employing an alkali hydroxide such as
potassium hydroxide or sodium hydroxide is effective for producing
an activated carbon material having a large specific surface area,
and an activated carbon material produced through this activation
method exhibits high capacitance per mass or per volume.
When alkali activation, for example, activation employing an alkali
hydroxide, is employed, an alkali metal compound is melted through
heating, and a carbon material is impregnated and reacted with the
molten alkali metal compound, to thereby form a porous structure
and activate the carbon material. When alkali activation of powdery
or granular carbon raw material is employed in a container such as
a crucible, effervescence of a molten liquid occurs during
activation, due to generation of, for example, moisture or hydrogen
gas, and the molten liquid may overflow the container. Particularly
when alkali activation is carried out at high temperature increase
rate, the amount of gas generated in a unit time increases, and
overflow of the molten liquid tends to occur. In order to avoid
such overflow, the amounts of an alkali metal compound and a carbon
raw material which are placed in a container must be limited, and
therefore productivity of an alkali-activated carbon material is
considerably lowered, resulting in high production cost.
Accordingly, an object of the present invention is to provide an
activated carbon material which enables capacitance per electrode
to be increased without application of a high voltage.
Another object of the present invention is to drastically improve
the productivity in the activation of an activated carbon material
and to produce an activated carbon material which is excellent in
capacitance at high current density.
SUMMARY OF THE INVENTION
The present invention has been accomplished as a result of
extensive investigations for solving the aforementioned problems
and provides an activated carbon material, the production method
and the use thereof shown below. 1. A method for producing an
activated carbon material, wherein the method comprises a step of
thermally treating coal-based pitch at two temperature ranges of
400.degree. C. to 600.degree. C. and 600.degree. C. to 900.degree.
C.; and a step of mixing and heating the thus-treated coal-based
pitch with an alkali metal compound for the activation thereof. 2.
The method for producing an activated carbon material according to
1 above, wherein the alkali metal compound is at least one alkali
hydroxide selected from the group consisting of sodium hydroxide,
potassium hydroxide, and cesium hydroxide. 3. The method for
producing an activated carbon material according to 1 above,
wherein the step of thermally treating coal-based pitch at two
temperature ranges is carried out in a vapor of an alkali metal. 4.
The method for producing an activated carbon material according to
3 above, wherein the alkali metal compound is at least one species
selected from the group consisting of potassium, sodium, and cesium
compounds. 5. The method for producing an activated carbon material
according to 1 above, wherein the step for the activation comprises
adding a fibrous material to the coal-based pitch. 6. The method
for producing an activated carbon material according to 5 above,
wherein the amount of the fibrous material is not less than 0.05
mass % as a corresponding mass of the fibrous material heated at
800.degree. C. in an inert atmosphere on the basis of the
coal-based pitch. 7. The method for producing an activated carbon
material according to 5 or 6 above, wherein the outer diameter of
each fiber filament of the fibrous material is 1000 nm or less. 8.
The method for producing an activated carbon material according to
any one of 5 to 7 above, wherein the fibrous material is a material
capable of maintaining its shape up to at least 300.degree. C. 9.
The method for producing an activated carbon material according to
any one of 5 to 8 above, wherein the fibrous material is at least
one species selected from the group consisting of a fibrous carbon,
carbonized material of organic fiber, unmeltable fiber, beaten pulp
and cellulose fiber. 10. The method for producing an activated
carbon material according to 9 above, wherein the fibrous carbon is
at least one species selected from the group consisting of a carbon
nano tube, whiskers, vapor grown carbon fiber, carbon fiber ribbon
and coiled carbon fiber. 11. The method for producing an activated
carbon material according to 10 above, wherein each fiber filament
of the vapor grown carbon fiber contains a hollow space extending
along its center axis, and has an outer diameter of 2 to 500 nm and
an aspect ratio of 10 to 15,000. 12. The method for producing an
activated carbon material according to 11 above, wherein the vapor
grown carbon fiber is branched carbon fiber. 13. An activated
carbon material produced through a production method as recited in
any one of 1 to 12 above. 14. The activated carbon material
according to 13 above, which has a BET specific surface area of 10
to 1,000 m.sup.2/g as measured by means of a nitrogen adsorption
method, and contains no graphite microcrystals. 15. The activated
carbon material according to 13 or 14 above, wherein the ratio of
the height of the D peak (1,360 cm.sup.-1) of a Raman spectrum of
the activated carbon material to that of the G peak (1,580
cm.sup.-1) of the Raman spectrum is 0.8 to 1.2. 16. The activated
carbon material according to any one of 13 to 15 above, wherein
pores of the activated carbon material having a size of 20 to 50
.ANG. as measured by means of a BJH method employing nitrogen
adsorption have a pore volume of at least 0.02 ml/g. 17. A
polarizable electrode material comprising a activated carbon
material as recited in any one of 13 to 16 above and optionally
vapor grown carbon fiber. 18. The polarizable electrode material
according to 17 above, wherein the amount of the vapor grown carbon
fiber is 0.05 to 50 mass %. 19. The polarizable electrode material
according to 17 or 18 above, wherein each fiber filament of the
vapor grown carbon fiber contains a hollow space extending along
its center axis, and has an outer diameter of 2 to 500 nm and an
aspect ratio of 10 to 15,000. 20. The polarizable electrode
material according to any one of 17 to 19 above, wherein the vapor
grown carbon fiber contains micropores having a pore volume of 0.01
to 0.4 ml/g, and has a BET specific surface area of 30 to 1,000
m.sup.2/g as measured by means of a nitrogen adsorption method. 21.
An electric double layer capacitor comprising a polarizable
electrode prepared from a polarizable electrode material as recited
in any one of 17 to 20 above. 22. A method for producing an
activated carbon material, wherein the method comprises adding an
alkali metal compound as an activating agent and a fibrous material
to a carbonaceous raw material and heating the mixture. 23. The
method for producing an activated carbon material according to 22
above, wherein the amount of the fibrous material is not less than
0.05 mass % as a corresponding mass of the fibrous material heated
at 800.degree. C. in an inert atmosphere on the basis of the
carbonaceous raw material. 24. The method for producing an
activated carbon material according to 22 or 23 above, wherein the
outer diameter of each fiber filament of the fibrous material is
1000 nm or less. 25. The method for producing an activated carbon
material according to any one of 22 to 24 above, wherein the
fibrous material is a material capable of maintaining its shape up
to at least 300.degree. C. 26. The method for producing an
activated carbon material according to any one of 22 to 25 above,
wherein the fibrous material is at least one species selected from
the group consisting of a fibrous carbon, carbonized material of
organic fiber, unmeltable fiber, beaten pulp and cellulose fiber.
27. The method for producing an activated carbon material according
to 26 above, wherein the fibrous carbon is at least one species
selected from the group consisting of a carbon nano tube, whiskers,
vapor grown carbon fiber, carbon fiber ribbon and coiled carbon
fiber. 28. The method for producing an activated carbon material
according to 27 above, wherein each fiber filament of the vapor
grown carbon fiber contains a hollow space extending along its
center axis, and has an outer diameter of 2 to 500 nm and an aspect
ratio of 10 to 15,000. 29. The method for producing an activated
carbon material according to 28 above, wherein the vapor grown
carbon fiber is branched carbon fiber. 30. An activated carbon
material having a fibrous material fused onto at least a portion of
the surface of the activated carbon material particle. 31. The
activated carbon material according to 30 above, which assumes a
spherical shape. 32. An activated carbon material produced through
a method for producing an activated carbon material as recited in
any one of 22 to 29 above. 33. A polarizable electrode comprising,
as an electrode material, an activated carbon material as recited
in any one of 30 to 32 above. 34. An electric double layer
capacitor comprising a polarizable electrode as recited in 33
above.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view showing a cell employed for
evaluating an electric double layer capacitor.
FIG. 2 shows a Raman spectrum of the activated carbon material
produced in Example 1. The vertical axis corresponds to spectrum
intensity (Int.), and the horizontal axis corresponds to Raman
shift (measurement wavelength: cm.sup.-1).
FIG. 3 shows a transmission electron microscope (TEM) photograph of
the activated carbon material produced in Example 3 (magnification:
.times.2,000,000).
FIG. 4 shows an electron micrograph of the activated carbon
material produced in Example 6 (magnification: .times.5,000).
DESCRIPTION OF THE INVENTION
Electric characteristics of an activated carbon material greatly
vary with structural characteristics, including specific surface
area, pore distribution, and crystal structure of the activated
carbon material. Such structural characteristics of the activated
carbon material are determined on the basis of the structure of a
raw material, carbonization conditions, and activation
conditions.
Therefore, in order to produce an activated carbon material useful
as an electrode material, the structure of a raw material,
carbonization conditions, and activation conditions must be
optimized. The present inventors have considered that coal-based
pitch is suitably employed as a raw material for producing an
activated carbon material. As compared with a petroleum-based
carbon raw material, coal-based pitch has a small number of side
chains, contains aromatic compounds at high proportions, and
contains polycyclic aromatic compounds of different molecular
structures. Therefore, when an activated carbon material is
produced from coal-based pitch, conceivably, a variety of
complicated microcrystalline structures derived from such compounds
are formed in the activated carbon material, and thus the activated
carbon material exhibits excellent electric characteristics.
No particular limitation is imposed on the coal-based pitch which
may be employed. However, coal-based pitch having a softening point
of 100.degree. C. or lower is preferred, and coal-based pitch
having a softening point of 60.degree. C. to 90.degree. C. is more
preferred.
Such coal-based pitch is subjected to two-stage heat treatment
including firing and carbonization at temperature ranges of 400 to
600.degree. C. and 600 to 900.degree. C., preferably 450 to
550.degree. C. and 650 to 850.degree. C.
When coal-based pitch is heated at 400 to 600.degree. C., thermal
decomposition reaction proceeds, gas and light components are
removed from the pitch, polycondensation of the residue occurs, and
finally the pitch is solidified. In this first-stage carbonization
step, the state of microscopic bonding between carbon atoms is
substantially determined, and the crystalline structure determined
in this step determines the fundamental structure of an activated
carbon material (i.e., a final product).
In the first-stage carbonization step, the temperature increase
rate is preferably 3 to 10.degree. C./hour, more preferably 4 to
6.degree. C./hour; and the maintenance time at the maximum
temperature is preferably 5 to 20 hours, more preferably 8 to 12
hours.
Subsequently, the second-stage heat treatment is carried out at a
temperature range of 600 to 900.degree. C. In this second-stage
carbonization step, the temperature increase rate is preferably 3
to 10.degree. C./hour, more preferably 4 to 6.degree. C./hour; and
the maintenance time at the maximum temperature is preferably 5 to
20 hours, more preferably 8 to 12 hours.
The above heat treatment (carbonization) steps are effectively
carried out in a vapor of an alkali metal. An alkali metal serves
as a catalyst in the carbonization step. That is, an alkali metal
promotes cross-linking between aromatic compounds contained in the
pitch, to thereby allow carbonization to proceed. Examples of the
alkali metals include compounds of sodium, potassium and
cesium.
The heat treatment method in a vapor of an alkali metal can be
conducted, for example, by heating the carbonization step system
while introducing to the system a vapor of an alkali metal
vaporized from the alkali activation reaction system described
below. Alternatively, the heat treatment step can be conducted by
placing pitch material around the vessel for the reaction alkali
activation reaction to expose the pitch material to an alkali metal
vapor vaporized from the alkali activation reaction system while
heating the pitch material, thereby concurrently effecting the heat
treatment (carbonization) and alkali activation reaction steps.
This method shortens the total treatment time and also reduces the
cost for heating.
Subsequently, the carbonized material (thermally treated
carbonaceous material) is subjected to pulverization so as to
attain a particle size of about 1 to about 100 .mu.m, and the
thus-pulverized product is mixed with an alkali metal compound and
then heated so as to form pores in the carbonized product, thereby
producing an activated carbon material.
In an activation method employing an alkali metal compound (an
alkali activation method), a carbonaceous raw material (e.g., a
carbonized material) is uniformly impregnated with an alkali metal
compound, and the carbonaceous raw material is heavily corroded by
the alkali metal compound under heating (firing), to thereby
produce an activated carbon material having an intricately
developed porous structure.
The activation step is preferably conducted with a fibrous material
mixed with the raw material. Expansion of an alkali molten liquid
can be suppressed by mixing a fibrous material, which enables the
productivity to be improved. Furthermore, the fibrous carbon
exhibiting excellent electrical conductivity (e.g., vapor grown
carbon fiber) is melt-bonded onto the surfaces of the resultant
activated carbon particles, and contact resistance between the
activated carbon particles can be reduced. As a result, when the
activated carbon is used as a polarizable electrode for an electric
double layer capacitor, properties such as capacitance holding
properties (cycle properties) thereof are improved.
Similar effects can be obtained by using other carbonaceous
materials in place of a coal-based pitch as a carbonaceous raw
material in the alkali activation method comprising mixing of a
fibrous material.
No particular limitation is imposed on the other carbonaceous
material employed in the production method of the present
invention. Examples of the carbonaceous raw material which may be
employed include products obtained through carbonization of coconut
shell, coffee bean, lignin, sawdust, polyvinylidene chloride,
phenol resin, coal, coal tar, coal coke, and petroleum coke;
PAN-based carbon fiber; and pitch-based carbon fiber. Carbonization
of a carbonaceous raw material can be carried out in a method other
than two-stage heating. A carbonaceous raw material is carbonized
typically at 400 to 1,000.degree. C. but a material which has not
undergone carbonization may be mixed with an alkali metal compound.
When a carbonaceous raw material is carbonized at a temperature
higher than 1,000.degree. C., the activation rate of the resultant
material is lowered, and activation of the material requires a long
period of time. The particle size of a carbonaceous raw material
may be that of a 10-mesh sieve (ASTM standards, mesh size: 2.0 mm)
or rougher one. However, the particle size of a carbonaceous raw
material is preferably that of a residue obtained through sieving
by use of a sieve of 10 mesh or less, more preferably, 50 mesh
(mesh size: 0.297 mm) or less, much more preferably 100 mesh (mesh
size: 0.149 mm) or less.
No particular limitation is imposed on the fibrous carbon usable in
the present invention, so long as the fibrous carbon can maintain
its shape up to 300.degree. C. and hold an alkali molten liquid.
Examples of the fibrous carbon which may be employed include a
fibrous carbon (e.g. carbon nano tube, whiskers, vapor grown carbon
fiber, carbon fiber ribbon and coiled carbon fiber), beaten pulp
products, cellulose fibers (e.g. natural fiber, regenerated
cellulose), carbonized material of organic fiber (e.g., PAN) and
unmeltable fiber. These fibers may be employed in combination of
two or more species. The term "unmeltable fiber" as used herein
refers to a fiber made from a spun fiber such as melt spinning
fiber, centrifugal spinning fiber and the like which has been
subjected to a heat treatment in an oxidizing atmosphere such as
air and oxygen thereby provided with bridges between fiber
constituting molecules and thus prevented from getting out of its
shape during the subsequent heat treatment.
No particular limitation is imposed on the shape of the fibrous
carbon, so long as the fibrous carbon can suppress expansion of an
alkali molten liquid during activation. Examples of the shape of
the fibrous carbon include a ribbon-like shape which has a flat
cross section, a coiled shape (e.g. coil, spiral, helix and
spring), carbon spring, carbon microcoil, helical polyacetylene and
the like.
The reason why expansion of an alkali molten liquid can be
suppressed by addition of fibrous carbon has not yet been fully
elucidated. Conceivably, an alkali molten liquid is held in fibrous
carbon. Branched fibrous carbon of small diameter exerts the effect
of suppressing expansion of an alkali molten liquid. The outer
diameter of fibrous carbon to be employed is typically 1000 nm or
less, preferably 500 nm or less, more preferably 10 nm or more and
400 nm or less. Those which have been fibrilized (i.e. made into
fibrils) such as beaten pulp (cellulose fibers mechanically
squashed or cut in water or the like) can be also used. In this
case, the outer diameter of the primary fiber can be 10 .mu.m or
more as long as that of each fibril is 1 .mu.m (1000 nm) or
less.
The amount of fibrous material added to a carbon raw material may
be determined in consideration of intended physical properties of a
final product, production cost, the effect of suppressing expansion
of a molten liquid depending on the shape of the fibrous carbon,
dispersibility of the fibrous carbon, the relation between
temperature increase rate and the degree of expansion of the molten
liquid. In the case that an organic fiber such as pulp and the like
is used, decrease in the mass thereof may occur during the process
due to carbonization and accordingly, the amount of the fiber to be
added is more than that in case of fibrous carbon. Typically, it is
sufficient that the amount of the fibrous material is not less than
0.05 mass % as a corresponding mass of the fibrous material heated
at 800.degree. C. in an inert atmosphere for 60 to 600 minutes.
When the amount of the fibrous material is too low, expansion of
the molten liquid cannot be sufficiently suppressed. Addition of a
fibrous material having an excellent conductivity is more
advantageous since such a fiber lowers the contact resistance
between the resulted activated carbon particles.
As a conductive fiber, vapor grown carbon fiber is preferably used
since the vapor grown carbon fiber contains carbon crystals grown
along the axis of each fiber filament of the fiber and contact
resistance between activated carbon particles can be effectively
reduced. Vapor grown carbon fiber can be produced by feeding a
gasified organic compound such as benzene into a high-temperature
atmosphere together with a transition metallic compound serving as
a catalyst.
The vapor grown carbon fiber to be employed may be as-produced
carbon fiber; carbon fiber which has undergone heat treatment at,
for example, 800 to 1,500.degree. C.; or carbon fiber which has
undergone graphitization at, for example, 2,000 to 3,000.degree. C.
However, as-produced carbon fiber or carbon fiber which has
undergone heat treatment at about 1,500.degree. C. is more
preferred.
Preferably, the vapor grown carbon fiber employed in the present
invention is branched carbon fiber. More preferably, each fiber
filament of the branched carbon fiber has a hollow structure in
which a hollow space extends throughout the filament, including a
branched portion thereof, sheath-forming carbon layers of the
filament assume uninterrupted layers. As used herein, the term
"hollow structure" refers to a structure in which a plurality of
carbon layers form a sheath. The hollow structure encompasses a
structure in which sheath-forming carbon layers form an incomplete
sheath; a structure in which the carbon layers are partially
broken; and a structure in which the laminated two carbon layers
are formed into a single carbon layer. The cross section of the
sheath does not necessarily assume a round shape, and may assume an
elliptical shape or a polygonal shape. No particular limitation is
imposed on the interlayer distance (d002) of carbon crystal layers.
The interlayer distance (d002) of the carbon layers as measured
through X-ray diffraction is preferably 0.339 nm or less, more
preferably 0.338 nm or less. The thickness (Lc) of the carbon
crystal layer in the C axis direction is 40 nm or less.
The outer diameter of each fiber filament of the vapor grown carbon
fiber is 2 to 500 nm, and the aspect ratio of the filament is 10 to
15,000. Preferably, the fiber filament has an outer diameter of 50
to 500 nm and a length of 1 to 100 .mu.m (i.e., an aspect ratio of
2 to 2,000); or an outer diameter of 2 to 50 nm and a length of 0.5
to 50 .mu.m (i.e., an aspect ratio of 10 to 25,000).
When the carbon material is activated with the alkali metal
compound, a portion of low crystallinity contained in the surface
of the carbon material is corroded by the alkali metal compound. In
the case where the carbon fiber is mixed with the carbon material,
while the surface of the carbon material is corroded, the carbon
fiber which is present in the vicinity of the carbon material is
melt-bonded to the carbon material, and thus, the carbon fiber is
melt-bonded onto the surfaces of the resultant activated carbon
particles. Such melt-bonding of carbon fibers onto the surface of
the activated carbon material reduces the contact resistance
between the activated carbon particles and improves the capacitance
at a high current density.
The melt-bonded state include not only such a state where the
carbonaceous surface layer of a fibrous material, for example,
carbon fiber and that of an activated carbon material are molten
and bonded to each other at points but also a state where those
surface layers are not molten but bonded to each other at their
solid surfaces.
The amount of carbon fiber mixed with the carbon material is
preferably not less than 0.05 mass %, more preferably 0.1 to 50
mass %, most preferably 1 to 30 mass %. When the amount of the
carbon fiber is less than 0.05 mass %, since the amount of the
melt-bonded carbon fiber is small, the effect of reducing the
contact resistance between the activated carbon particles is also
small, the improvement in charge/discharge characteristics at high
current density are not sufficiently obtained, and the effect of
suppressing the expansion the molten liquid is small.
When the amount of carbon fiber to be mixed with is smaller than
the mixing ratio of the conductive material to the activated carbon
for forming an electric double-layer capacitor, additional amount
of carbon fiber may be added to the resulted activated carbon to
form a capacitor. Alternatively, commonly used conductive material
such as carbon black may be added.
In contrast, when the amount of carbon fiber to be mixed with is
larger than the normal mixing ratio of the conductive material to
the activated carbon for forming an electric double-layer
capacitor, suitable amount of carbon fiber may be added to a common
activated carbon for enhancing electric conductivity to form an
electric double-layer capacitor.
For example, when the carbon fiber is added in an amount of 0.05 to
10 mass % and the activation is effected, the product may be used
for a polarizable electrode as it is, or alternatively, carbon
fiber and/or carbon black may be further added to the product to
form a polarizable electrode. When the amount of carbon fiber added
exceeds 50 mass %, the activated product may be added in an amount
of 10 to 50 mass % of to 90 to 50 mass % of the activated carbon to
form a polarizable electrode.
No particular limitation is imposed on the alkali activation agent,
so long as the agent is a compound containing an alkali metal. The
present invention is effectively applied to substances which melt
during activation. Preferred examples of the alkali activation
agent include hydroxides, carbonates, sulfides, and sulfates of
potassium, sodium, and calcium. Specific examples of the alkali
activation agent which may be employed include potassium hydroxide,
sodium hydroxide, cecium hydroxide, potassium carbonate, sodium
carbonate, potassium sulfide, sodium sulfide, potassium
thiocyanate, potassium sulfate, and sodium sulfate. Potassium
hydroxide and sodium hydroxide are preferred, and potassium
hydroxide is more preferred. These compounds may be employed singly
or in combination of two or more species.
The amount of an alkali metal compound mixed with a carbonaceous
raw material may be determined in accordance with crystallinity of
the carbonaceous raw material, the amount of a surface functional
group of the carbonaceous raw material, and the intended use of an
activated carbon material to be produced. When crystallinity of the
carbonaceous raw material is high and the amount of a surface
functional group of the raw material is small, the amount of an
alkali metal compound which is required tends to increase. For
example, when potassium hydroxide is employed, the ratio by mass of
potassium hydroxide to the carbonaceous raw material is about 0.5
to about 7, preferably about 1 to about 5, more preferably about 2
to about 4. When the ratio by mass of potassium hydroxide is less
than 0.5, micropores are insufficiently developed, whereas when the
ratio by mass is 7 or more, excessive activation proceeds and the
walls of micropores are broken, so that the number of micropores
decreases, and thus the specific surface area of the resultant
activated carbon material tends to be reduced.
The activation temperature varies with the type and shape of a raw
material and the activation reaction rate. The activation
temperature is typically 250 to 1,000.degree. C., preferably
500.degree. C. to 900.degree. C., more preferably 600.degree. C. to
800.degree. C. When the activation temperature is 400.degree. C. or
lower, activation proceeds insufficiently, the number of micropores
contained in an activated carbon material becomes small, and
capacitance when used as a polarizable electrode material in an
electric double layer capacitor is lowered. When the activation
temperature is 1,000.degree. C. or higher, there arise problems,
including shrinkage of micropores contained in an activated carbon
material, considerable deterioration of charge characteristics at
high current density, and corrosion of an activation apparatus.
The temperature increase rate during activation may be determined
whether fibrous carbon is added or not or in consideration of the
amount of fibrous carbon to be added, and the degree of expansion
of an alkali molten liquid. When the temperature increase rate is
high, the amount of moisture removed from an alkali metal compound
per unit time increases, and the amount of gas (e.g., hydrogen gas)
generated from the alkali metal compound per unit time increases,
and thus the alkali metal compound tends to overflow a container.
In contrast, when the temperature increase rate is low, the yield
of an activated carbon material per container increases, but
productivity is lowered. The temperature increase rate is typically
400 to 1.degree. C./hour.
For example, when the temperature increase rate is 300.degree.
C./hour, the total volume of a carbonaceous raw material, fibrous
carbon, and an alkali metal compound to be placed in a container
(crucible) is 15% that of the entire volume of the container. When
the temperature increase rate is lowered to 20.degree. C./hour or
less, the amount of gas generated per unit time can be reduced, and
the yield of an activated carbon material per container can be
increased. However, from the viewpoint of productivity, preferably,
the total amount of a carbonaceous raw material, fibrous carbon,
and an alkali metal compound to be placed in a container is
increased to a possibly maximum level, and activation is carried
out at a high temperature increase rate.
As a result of activation, numerous basket-like micropores having a
size of 2 to 5 nm (20 to 50 .ANG.) are formed between several
carbon layers, the number of micropores which has a radius of about
1 to about 2 nm (about 10 to about 20 .ANG.) and which is suitable
for, for example, adsorption is increased, and the adsorption
volume of an activated carbon material is increased.
The thus-produced activated carbon material exhibits high
capacitance at the first cycle of charge/discharge testing without
application of excessive voltage, and exhibits high percent
maintenance of the capacitance.
When the activated carbon material was observed under a
transmission microscope, the material was found to contain no
microcrystals similar to those of graphite, and to have merely a
turbostratic structure as shown in FIG. 3. The ratio of the height
of the D peak (1360 cm.sup.-1) of a Raman spectrum of the material
to the height of the G peak (1580 cm.sup.-1) of the Raman spectrum
(i.e., height from the base line to the peak point as measured on
the basis of the spectrum) in an observed curve was found to be 0.8
to 1.2.
Here, the ratio of the intensity of the D peak to that of the G
peak is used as an indicator to indicate the graphitization degree
of carbon material, and the intensity ratio expressed as the peak
height ratio becomes a smaller value as the graphitization degree
is larger. The value in an activated carbon containing
microcrystals is generally on the order of 0.6 and the present
activated carbon containing no microcrystals exhibited values of
0.8 to 1.2.
Furthermore, an activated carbon black to be used in an electrode
of a capacitor is required to have micropores of 20 to 50 .ANG.,
which is considered to attribute to the capacitance and the
expansion of the electrolyte, in a certain amount of more.
The BET specific surface area of the activated carbon material of
the present invention determined by nitrogen adsorption method was
found to be 10 to 1,000 m.sup.2/g, which is smaller than that of
the conventionally obtained activated carbon materials (usually in
the range of 2,000 to 3,000 m.sup.2/g). The pore volume of pores
having a size of 20 to 50 .ANG. of the activated carbon material of
the present invention as measured by means of a BJH (Barrett,
Joyner and Halenda) method was found to be 0.02 ml/g or more.
Due to the crystalline structure and micropore structure, the
activated carbon material exhibits a high capacitance from the
first charge/discharge cycle without application of excessive
voltage for intercalation of ions between graphite layers. In
addition, conceivably, since the activated carbon material has
undergone sufficient carbonization, the amount of functional groups
on the surface of the material is reduced, and lowering of
capacitance can be prevented.
The tapping density of the thus produced activated carbon material
was measured by use of a tapping density meter (product of
Kuramochi Kagaku Kikai Seisakusho), and found to be 0.35 to 0.70
g/ml (tapping: 50 times). The powder resistance of the activated
carbon material was found to be 0.4 .OMEGA.cm or less at 1.0
MPa.
When vapor grown carbon fiber is added to the thus-produced
activated carbon material, characteristics of the activated carbon
material are further improved. Similar vapor grown carbon fiber as
described above can be employed for that purpose. When the alkali
activation has been effected after the addition of vapor grown
carbon fiber, vapor grown carbon fiber can be added if
required.
When the activated carbon material is mixed with vapor grown carbon
fiber, contact resistance between activated carbon particles is
reduced. Therefore, when a polarizable electrode is formed from the
resultant mixture, the electrode exhibits enhanced strength and
improved durability.
Vapor grown carbon fiber to be employed may be as-produced carbon
fiber which has undergone firing at 1,000 to 1,500.degree. C., or
carbon fiber which has undergone firing and then
graphitization.
Vapor grown carbon fiber which has undergone gas activation or
chemical activation may be employed. When such vapor grown carbon
fiber is employed, preferably, the surface of the carbon fiber may
be controlled such that micropores (i.e., pores having a size of 20
.ANG. or less) contained in the carbon fiber have a pore volume of
0.01 to 0.4 ml/g, and the carbon fiber has a BET specific surface
area of 30 to 1,000 m.sup.2/g. When carbon fiber containing a large
number of micropores is mixed with the activated carbon material,
ion diffusion resistance in an electrode formed from the resultant
mixture increases.
The amount of vapor grown carbon fiber mixed with the activated
carbon material is preferably 0.02 mass % to 50 mass %, more
preferably 0.05 to 30 mass %. When the amount of the carbon fiber
is 0.02 mass % or less, since contact points between the carbon
fiber and activated carbon particles increase insufficiently,
satisfactory effects fail to be obtained. In contrast, when the
amount of the carbon fiber is 50 mass % or more, the activated
carbon content of a polarizable electrode is lowered, resulting in
lowering of capacitance.
A polarizable electrode and an electric double layer capacitor can
be produced from the activated carbon material of the present
invention through any known method. Specifically, a polarizable
electrode is produced through, for example, the following methods:
a method in which an electrically conductive agent and a binder are
added to the activated carbon material, and the resultant mixture
is subjected to kneading and rolling; a method in which an
electrically conductive agent, a binder, and if desired, a solvent
are added to the activated carbon material to thereby prepare a
slurry, and the resultant slurry is applied in a predetermined
thickness to an electrically conductive material followed by
removing the solvent by evaporation at room or elevated
temperature; and a method in which a non-carbonized resin is mixed
with the activated carbon material, and the resultant mixture is
sintered. Examples of the electrically conductive material usable
for this purposes include a foil or a plate of aluminum,
carbon-coated aluminum, stainless steel, titanium and the like
having a thickness of about 10 .mu.m to about 0.5 mm.
For example, if desired, an electrically conductive agent (e.g.,
carbon black) is added to the powdery activated carbon material
having an average particle size of about 5 to about 100 .mu.m; a
binder such as polytetrafluoroethylene (PTFE) or polyvinylidene
fluoride is added to the resultant mixture; the resultant mixture
is formed into a sheet having a thickness of about 0.1 to about 0.5
mm; and the thus-formed sheet is dried under vacuum at a
temperature of about 100 to about 200.degree. C. Electrodes having
a predetermined shape are formed from the sheet through punching. A
metallic plate serving as a collector is laminated on each of the
electrodes, to thereby form a laminate. A separator is sandwiched
by the thus-formed two laminates such that the metallic plates are
positioned outside, and the resultant laminate is immersed in an
electrolytic solution, to thereby produce an electric double layer
capacitor.
Any known electrolytic solution containing a non-aqueous solvent or
any known water-soluble electrolytic solution may be employed as an
electrolytic solution for the electric double layer capacitor.
Examples of aqueous systems (aqueous electrolytic solutions)
include a sulfuric acid aqueous solution, a sodium sulfate aqueous
solution, a sodium hydroxide aqueous solution, a potassium
hydroxide aqueous solution, an ammonium hydroxide aqueous solution,
a potassium chloride aqueous solution, and a potassium carbonate
aqueous solution.
A preferred non-aqueous system (non-aqueous electrolytic solution)
is prepared from an organic solvent, and a quaternary ammonium salt
or a quaternary phosphonium salt (i.e., electrolyte) containing a
cation represented by R.sup.1R.sup.2R.sup.3R.sup.4N.sup.+ or
R.sup.1R.sup.2R.sup.3R.sup.4P.sup.+ (wherein each of R.sup.1,
R.sup.2, R.sup.3, and R.sup.4 represents a C1 C10 alkyl group or a
C1 C10 allyl group) and an anion such as BF.sub.4.sup.-,
PF.sub.6.sup.-, or ClO.sub.4.sup.-. Examples of the organic
solvents include ethers such as diethyl ether, dibutyl ether,
ethylene glycol monomethyl ether, ethylene glycol monoethyl ether,
ethylene glycol monobutyl ether, diethylene glycol monomethyl
ether, diethylene glycol monoethyl ether, diethylene glycol
monobutyl ether, diethylene glycol dimethyl ether, and ethylene
glycol phenyl ether; amides such as formamide, N-methylformamide,
N,N-dimethyiformamide, N-ethylformamide, N, N-diethylformamide,
N-methylacetamide, N,N-dimethylacetamide, N-ethylacetamide,
N,N-diethylacetamide, N,N-dimethylpropionamide, and
hexamethylphosphoryl amide; sulfur-containing compounds such as
dimethyl sulfoxide and sulfolan; dialkyl ketones such as methyl
ethyl ketone and methyl isobutyl ketone; cyclic ethers such as
ethylene oxide, propylene oxide, tetrahydrofuran,
2-methoxytetrahydrofuran, 1,2-dimethoxyethane, and 1,3-dioxolan;
carbonates such as ethylene carbonate and propylene carbonate;
.gamma.-butyrolactone; N-methylpyrrolidone; acetonitrile; and
nitromethane. Preferred examples which may be employed include
carbonate-based non-aqueous solvents such as ethylene carbonate and
propylene carbonate. These electrolytes or solvents may be employed
in combination of two or more species.
No particular limitation is imposed on the separator which, if
desired, is provided between electrodes, so long as the separator
is an ion-permeable porous separator. Preferred examples of
separators which may be employed include microporous polyethylene
film, microporous polypropylene film, polyethylene nonwoven fabric,
polypropylene nonwoven fabric, glass-fiber-mixed nonwoven fabric,
and glass mat filter.
MODE FOR CARRYING OUT THE INVENTION
The present invention will next be described in more detail by way
of illustrative examples, which should not be construed as limiting
the invention thereto. In the following examples, characteristics
of activated carbon materials and electric double layer capacitors
were evaluated by means of the below-described methods.
(1) Measurement of BET Specific Surface Area and Pore Volume
BET specific surface area and pore volume were calculated by means
of a BET method and a BJH method, on the basis of an adsorption
isotherm of nitrogen as measured at liquid nitrogen temperature by
use of NOVA1200 (product of Quantachrome Instruments). The amount
of adsorbed nitrogen was measured at a relative pressure (P/P0) of
0.01 to 1.0.
(2) Measurement of Raman Spectrum
The Raman spectrum of a carbon material serving as a raw material
for producing an activated carbon material was measured under the
following conditions: excitation light source: Ar laser (514.5 nm),
detector: charge coupled device (CCD), slit width: 500 .mu.m,
exposure time: 60 seconds.
(3) Capacitance
Polytetrafluoroethylene (PTFE) (10 parts by mass) and carbon black
(10 parts by mass) were added to an activated carbon material
having an average particle size of 30 .mu.m (80 parts by mass), the
resultant mixture was kneaded in an agate mortar, and the
thus-kneaded product was subjected to rolling by use of a roller,
to thereby form a sheet having a thickness of 0.5 mm. The
thus-formed sheet was subjected to punching to thereby form a disk
having a diameter of 20 mm, and the disk was dried at 200.degree.
C. under vacuum overnight. The resultant disk was employed as a
polarizable electrode.
A cell for evaluation as shown in FIG. 1 was assembled from the
aforementioned electrode in a glove box through which argon of high
purity was circulated. In FIG. 1, reference numeral 1 represents an
upper lid formed of aluminum, 2 an O ring formed of fluorine
rubber, 3 collectors formed of aluminum, 4 an insulator formed of
Teflon (registered trademark), 5 a container formed of aluminum, 6
a leaf spring formed of aluminum, 7 polarizable electrodes, and 8 a
separator formed of glass fiber (thickness: 1 mm). LIPASTE-P/EAFIN
(product of Tomiyama Pure Chemical Industries, Ltd.) (1 mol/liter),
containing propylene carbonate (PC) serving as a solvent and
(C.sub.2H.sub.5).sub.4NBF.sub.4 serving as an electrolyte, was
employed as an electrolytic solution.
Charging and discharging were carried out at a current of 5 mA (1.6
mA/cm.sup.2), 50 mA (16 mA/cm.sup.2) and 150 mA (48 mA/cm.sup.2)
and 0 to 2.5 V or 0 to 3.0 V by use of a charge/discharge test
apparatus (HJ-101SM6, product of Hokuto Denko Co., Ltd.). A
discharge curve obtained through the second constant-current
discharging was used to calculate capacitance (F/g) of an electric
double layer capacitor per mass of activated carbon contained in
the electrodes of the capacitor, along with capacitance (F/ml) of
the capacitor per volume of the activated carbon.
Durability was evaluated on the basis of percent maintenance of
capacitance (i.e., the ratio of capacitance after 20-cycle
charge/discharge testing to capacitance after the second-cycle
charging/discharging).
EXAMPLE 1
Coal pitch (product of Kawasaki Steel Corporation) having a
softening point of 86.degree. C. was subjected to first-stage heat
treatment at 500.degree. C. and then to second-stage heat treatment
at 700.degree. C. The resultant carbonaceous material was mixed
with KOH such that the ratio by mass of the KOH to the coal pitch
was 2.5, and the resultant mixture was placed in a crucible. The
crucible was heated to 750.degree. C. at 3.degree. C./hour, and the
temperature of the crucible was maintained at 750.degree. C. for 60
minutes, to thereby allow activation of the pitch to proceed. The
thus-activated carbon material was washed with 1N hydrochloric
acid, and then washed with distilled water, to thereby remove
residual KOH and metallic impurities. The thus-washed carbon
material was dried under vacuum at 200.degree. C., to thereby
produce an activated carbon material.
The specific surface area of the activated carbon material was
found to be 930 m.sup.2/g. The pore volume of pores of the
activated carbon material having a size of 20 to 50 .ANG. as
measured by means of a BJH method was found to be 0.0416 ml/g. FIG.
2 shows a Raman spectrum of the activated carbon material. The
ratio of the height of the D peak to that of the G peak was found
to be 0.92.
When charging and discharging were performed at a current of 5 mA
(1.6 mA/cm.sup.2) and 2.5 V, the capacitance was found to be 36.5
F/g and 31.0 F/ml, and the percent maintenance of capacitance after
20-cycle charging/discharging was found to be 98.4%. When charging
and discharging were performed at a current of 5 mA (1.6
mA/cm.sup.2) and 3.0 V, the capacitance was found to be 37.7 F/g
and 32.0 F/ml, and the percent maintenance of capacitance after
20-cycle charging/discharging was found to be 96.9%.
EXAMPLE 2
The activated carbon material produced through the method of
Example 1 was mixed with vapor grown carbon fiber (5 mass %), to
thereby prepare a polarizable electrode material. When charging and
discharging were performed at a current of 5 mA (1.6 mA/cm.sup.2)
and 2.5 V, the capacitance was found to be 36.4 F/g and 32.4 F/ml,
and the percent maintenance of capacitance after 20-cycle
charging/discharging was found to be 98.9%. When charging and
discharging were performed at a current of 5 mA (1.6 mA/cm.sup.2)
and 3.0 V, the capacitance was found to be 39.5 F/g and 35.2 F/ml,
and the percent maintenance of capacitance after 20-cycle
charging/discharging was found to be 97.7%.
EXAMPLE 3
The procedure of Example 1 was repeated, except that coal pitch
(product of Kawasaki Steel Corporation) having a softening point of
86.degree. C. was subjected to heat treatment at 500.degree. C. and
800.degree. C., to thereby produce an activated carbon
material.
The activated carbon material was employed as a polarizable
electrode material. The specific surface area of the activated
carbon material was found to be 173 m.sup.2/g. The pore volume of
pores of the activated carbon material having a size of 20 to 50
.ANG. as measured by means of a BJH method was found to be 0.0271
ml/g. The ratio of the height of the D peak of a Raman spectrum of
the activated carbon material to that of the G peak of the Raman
spectrum was found to be 0.93.
The activated carbon material was observed under a transmission
electron microscope (TEM) as shown in FIG. 3 and the activated
carbon material was found to have no graphite structure and to have
merely a turbostratic structure.
When charging and discharging were performed at a current of 5 mA
(1.6 mA/cm.sup.2) and 2.5 V, the capacitance was found to be 32.6
F/g and 31.9 F/ml, and the percent maintenance of capacitance after
20-cycle charging/discharging was found to be 98.7%. When charging
and discharging were performed at a current of 5 mA (1.6
mA/cm.sup.2) and 3.0 V, the capacitance was found to be 35.5 F/g
and 34.8 F/ml, and the percent maintenance of capacitance after
20-cycle charging/discharging was found to be 97.2%.
EXAMPLE 4
The activated carbon material produced through the method of
Example 3 was mixed with vapor grown carbon fiber which had
undergone alkali activation (pore volume of micropores: 0.3 ml, BET
specific surface area: 530 m.sup.2/g) (5 mass %), to thereby
prepare a polarizable electrode material. When charging and
discharging were performed at a current of 5 mA (1.6 mA/cm.sup.2)
and 2.5 V, the capacitance was found to be 33.5 F/g and 33.5 F/ml,
and the percent maintenance of capacitance after 20-cycle
charging/discharging was found to be 99.0%. When charging and
discharging were performed at a current of 5 mA (1.6 mA/cm.sup.2)
and 3.0 V, the capacitance was found to be 34.5 F/g and 34.5 F/ml,
and the percent maintenance of capacitance after 20-cycle
charging/discharging was found to be 98.0%.
COMPARATIVE EXAMPLE 1
Petroleum coke serving as a carbon material was mixed with KOH and
placed into a crucible such that the ratio by mass of the KOH to
the coke was 2.5 and the mixture was held at 750.degree. C. for 60
minutes to effect activation. The activated carbon material was
washed with 1N hydrochloric acid and then with distilled water to
remove remaining KOH and metallic impurities. This was vacuum dried
at 200.degree. C. to obtain activated carbon. The specific surface
area of the activated carbon material was found to be 1,905
m.sup.2/g. The ratio of the height of the D peak of a Raman
spectrum of the activated carbon material to that of the G peak of
the Raman spectrum was found to be 0.98.
When charging and discharging were performed at a current of 5 mA
(1.6 mA/cm.sup.2) and 2.5 V, the capacitance was found to be 44.5
F/g and 24.0 F/ml, and the percent maintenance of capacitance after
20-cycle charging/discharging was found to be 96.3%. When charging
and discharging were performed at a current of 5 mA (1.6
mA/cm.sup.2) and 3.0 V, the capacitance was found to be 45.0 F/g
and 24.3 F/ml, and the percent maintenance of capacitance after
20-cycle charging/discharging was found to be 94.0%.
COMPARATIVE EXAMPLE 2
MCMB (mesocarbon microbeads, product of Osaka Gas Co., Ltd.)
serving as a carbon material was mixed with KOH and placed into a
crucible such that the ratio by mass of the KOH to the MCMB was 5
and the mixture was held at 750.degree. C. for 60 minutes to effect
activation. The activated carbon material was washed with 1N
hydrochloric acid and then with distilled water to remove remaining
KOH and metallic impurities. This was vacuum dried at 200.degree.
C. to obtain activated carbon. The specific surface area of the
activated carbon material was found to be 127 m.sup.2/g. The pore
volume of pores of the activated carbon material having a size of
20 to 50 .ANG. was found to be 0.013 ml/g. The ratio of the height
of the D peak of a Raman spectrum of the activated carbon material
to that of the G peak of the Raman spectrum was found to be
0.92.
When charging and discharging were performed at a current of 5 mA
(1.6 mA/cm.sup.2) and 2.5 V, the capacitance was found to be 10.2
F/g and 9.4 F/ml, and the percent maintenance of capacitance after
20-cycle charging/discharging was found to be 99.1%. When charging
and discharging were performed at a current of 5 mA (1.6
mA/cm.sup.2) and 3.0 V, the capacitance was found to be 11.5 F/g
and 10.6 F/ml, and the percent maintenance of capacitance after
20-cycle charging/discharging was found to be 98.5%.
EXAMPLE 5
Coal pitch (product of Kawasaki Steel Corporation) having a
softening point of 86.degree. C. was subjected to first-stage heat
treatment at 500.degree. C. and then to second-stage heat treatment
at 700.degree. C. The resultant carbonaceous material was mixed
with KOH such that the ratio by mass of the KOH to the coal pitch
was 2.5 and with vapor grown carbon fiber (fiber diameter: 50 500
nm, fiber length: about 20 .mu.m) such that the ratio by mass of
the carbon fiber to the coal pitch was 0.05, and the resultant
mixture was placed in a crucible up to a height of 80 mm from the
bottom. The crucible was heated to 750.degree. C. at a temperature
increase rate of 350.degree. C./hour, and the temperature of the
crucible was maintained at 750.degree. C. for 60 minutes, to
thereby allow activation of the pitch to proceed. The
thus-activated carbon material was washed with 1N hydrochloric
acid, and then washed with distilled water, to thereby remove
residual KOH and metallic impurities. The thus-washed carbon
material was dried under vacuum at 200.degree. C., to thereby
produce an activated carbon material.
In a case where vapor grown carbon fiber was not used, an alkali
molten liquid rose up to a height of 560 mm from the bottom of the
crucible. In this Example, by using vapor grown carbon fiber, an
alkali molten liquid rose up no higher than a height of 120 mm (1.5
times as high as the filling height of the mixture) from the bottom
of the crucible, and productivity was drastically improved.
The specific surface area of the activated carbon material was
found to be 930 m.sup.2/g. The pore volume of pores of the
activated carbon material having a size of 20 to 50 .ANG. as
measured by means of a BJH method was found to be 0.0450 ml/g. The
ratio of the height of the D peak to that of the G peak was found
to be 0.90.
When charging and discharging were performed at a current of 5 mA
(1.6 mA/cm.sup.2) and 2.5 V, the capacitance was found to be 37.0
F/g and 31.5 F/ml, and the percent maintenance of capacitance after
20-cycle charging/discharging was found to be 99.5%. When charging
and discharging were performed at a current of 5 mA (1.6
mA/cm.sup.2) and 3.0 V, the capacitance was found to be 38.0 F/g
and 32.3 F/ml, and the percent maintenance of capacitance after
20-cycle charging/discharging was found to be 98.2%. As compared to
Example 1 where vapor grown carbon fiber was not used, the percent
maintenance of capacitance was improved.
EXAMPLE 6
A phenol resin (trade name: Bellpearl R800, product of Kanebo,
Ltd.) was carbonized in a nitrogen atmosphere at 700.degree. C. for
four hours. The thus-carbonized product (150 g), vapor grown carbon
fiber (average fiber diameter: about 500 nm, fiber length: about 20
.mu.m) (7.5 g), and potassium hydroxide pellets (473 g) were placed
in a metallic crucible (100 mm.phi..times.530 mm). The thickness of
a layer of the mixture of the carbonized product, vapor grown
carbon fiber, and potassium hydroxide was found to be 80 mm. The
crucible was placed in an electric furnace and heated to
750.degree. C. at a temperature increase rate of 350.degree. C./hr
under a nitrogen stream, and the temperature of the crucible was
maintained at 750.degree. C. for 30 minutes. After completion of
activation, the crucible was removed from the furnace, and then
visually observed. The results revealed that an alkali molten
liquid rose up to a height of 130 mm from the bottom of the
crucible. The thus-activated carbon material was washed with water
and 1N hydrochloric acid, and then washed with distilled water, to
thereby remove residual alkali and metallic impurities. After the
thus-washed activated carbon material was dried, BET specific
surface area and capacitance were measured. The BET specific
surface area was found to be 2,335 m.sup.2/g; and the capacitance
was found to be 42.9 F/g (at 1.6 mA/cm.sup.2), 36.7 F/g (at 16
mA/cm.sup.2), 24.7 F/g (at 48 mA/cm.sup.2), and 26.7 F/ml (at 1.6
mA/cm.sup.2).
The obtained activated carbon was observed under a transmission
electron microscope (TEM) as shown in FIG. 4 and the activated
carbon material was found to have a spherical shape on the surface
of which carbon fibers are melt-bonded.
EXAMPLE 7
A phenol resin (trade name: Bellpearl R800, product of Kanebo,
Ltd.) was carbonized in a nitrogen atmosphere at 700.degree. C. for
four hours. The thus-carbonized product (150 g), vapor grown carbon
fiber (average fiber diameter: about 500 nm, fiber length: about 20
.mu.m) (15 g), and potassium hydroxide pellets (495 g) were placed
in a metallic crucible (100 mm.phi..times.530 mm). The thickness of
a layer of the mixture of the carbonized product, vapor grown
carbon fiber, and potassium hydroxide was found to be 85 mm. The
crucible was placed in an electric furnace and heated to
750.degree. C. at a temperature increase rate of 350.degree. C./hr
under a nitrogen stream, and the temperature of the crucible was
maintained at 750.degree. C. for 30 minutes. After completion of
activation, the crucible was removed from the furnace, and then
visually observed. No rise of an alkali molten liquid was observed.
The thus-activated carbon material was washed with water and 1N
hydrochloric acid. After the thus-washed activated carbon material
was dried, BET specific surface area and capacitance were measured.
The BET specific surface area was found to be 2,400 m.sup.2/g; and
the capacitance was found to be 40.6 F/g (at 1.6 mA/cm.sup.2), 34.5
F/g (at 16 mA/cm.sup.2), 23.3 F/g (at 48 mA/cm.sup.2), and 25.9
F/ml (at 1.6 mA/cm.sup.2).
EXAMPLE 8
A phenol resin (trade name: Bellpearl R800, product of Kanebo,
Ltd.) was carbonized in a nitrogen atmosphere at 700.degree. C. for
four hours. The thus-carbonized product (150 g), wood beaten pulp
(48 g), and potassium hydroxide pellets (472 g) were placed in a
metallic crucible (100 mm.phi..times.530 mm). The thickness of a
layer of the mixture of the carbonized product, pulp, and potassium
hydroxide was found to be 170 mm. The crucible was placed in an
electric furnace and heated to 750.degree. C. at a temperature
increase rate of 350.degree. C./hr under a nitrogen stream, and the
temperature of the crucible was maintained at 750.degree. C. for 30
minutes. After completion of activation, the crucible was removed
from the furnace, and then visually observed. The results revealed
that an alkali molten liquid rose up to a height of 190 mm from the
bottom of the crucible. The thus-activated carbon material was
washed with water, 1N hydrochloric acid and then with distilled
water to thereby remove residual alkali and metallic impurities.
After the thus-washed activated carbon material was dried, BET
specific surface area and capacitance were measured. The BET
specific surface area was found to be 1,836 m.sup.2/g; and the
capacitance was found to be 32.8 F/g (at 1.6 mA/cm.sup.2) and 24.6
F/ml (at 1.6 mA/cm.sup.2).
EXAMPLE 9
A phenol resin (trade name: Bellpearl R800, product of Kanebo,
Ltd.) was carbonized in a nitrogen atmosphere at 700.degree. C. for
four hours. The thus-carbonized product (150 g), wood beaten pulp
carbonized at 700.degree. C. (7.5 g), and potassium hydroxide
pellets (472 g) were placed in a metallic crucible (100
mm.phi..times.530 mm). The thickness of a layer of the mixture of
the carbonized product, pulp, and potassium hydroxide was found to
be 75 mm. The crucible was placed in an electric furnace and heated
to 750.degree. C. at a temperature increase rate of 350.degree.
C./hr under a nitrogen stream, and the temperature of the crucible
was maintained at 750.degree. C. for 30 minutes. After completion
of activation, the crucible was removed from the furnace, and then
visually observed. The results revealed that an alkali molten
liquid rose up to a height of 170 mm from the bottom of the
crucible. The thus-activated carbon material was washed with, 1N
hydrochloric acid and then with distilled water to thereby remove
residual alkali and metallic impurities. After the thus-washed
activated carbon material was dried, BET specific surface area and
capacitance were measured. The BET specific surface area was found
to be 2,151 m.sup.2/g; and the capacitance was found to be 39.8 F/g
(at 1.6 mA/cm.sup.2) and 25.6 F/ml (at 1.6 mA/cm.sup.2).
COMPARATIVE EXAMPLE 3
A phenol resin (trade name: R800, product of Kanebo, Ltd.) was
carbonized in a nitrogen atmosphere at 700.degree. C. for four
hours. The thus-carbonized product (150 g) and potassium hydroxide
pellets (450 g) were placed in a metallic crucible (100
mm.phi..times.530 mm). The thickness of a layer of the mixture of
the carbonized product and potassium hydroxide was found to be 70
mm. The crucible was placed in an electric furnace and activation
was effected as in Example 6. After completion of activation, the
crucible was removed from the furnace, and then visually observed.
The results revealed that an alkali molten liquid rose up to a
height of 490 mm from the bottom of the crucible.
The thus-activated carbon material was washed with 1N hydrochloric
acid and then with distilled water to thereby remove residual
alkali and metallic impurities.
The product was evaluated as in Example 6. The capacitance was
found to be 39.4 F/g (at 1.6 mA/cm.sup.2), 31.7 F/g (at 16
mA/cm.sup.2) and 17.9 F/g (at 48 mA/cm.sup.2).
COMPARATIVE EXAMPLE 4
A phenol resin (trade name: R800, product of Kanebo, Ltd.) was
carbonized in a nitrogen atmosphere at 700.degree. C. for four
hours. To the thus-carbonized product (150 g) carbon fiber chop
(KrecaChop M-101S; fiber diameter: 14.5 .mu.m) (7.5 g) was added
and after potassium hydroxide pellets (473 g) were added thereto
the mixture was mixed well and placed in a inconel crucible (100
mm.phi..times.530 mm). The thickness of a layer of the mixture of
the carbonized product and potassium hydroxide was found to be 70
mm. The crucible was placed in an electric furnace and activation
was effected as in Example 6. After completion of activation, the
crucible was removed from the furnace, and then visually observed.
The results revealed that an alkali molten liquid rose up to a
height of 530 mm from the bottom of the crucible, and thus no
effect of suppressing the expansion of the molten liquid was
observed.
INDUSTRIAL APPLICABILITY
The method of the present invention comprising two-stage heat
treatment of a coal-based pitch at different temperature ranges and
activating with an alkali enables production of an activated carbon
material exhibiting excellent durability and high capacitance
(F/ml) without application of excessive voltage.
When the activated carbon material is mixed with vapor grown carbon
fiber, a polarizable electrode and an electric double layer
capacitor exhibiting more excellent characteristics can be
produced.
By adding fibrous carbon to a reactant (i.e., a composition
containing the carbonaceous raw material and the alkali metal
compound), expansion of an alkali molten liquid can be suppressed
during activation, and productivity can be improved.
Furthermore, employment of an fibrous carbon material which is
excellent in conductivity as a fibrous material, activated carbon
material on which carbon fiber is melt-bonded can be produced,
which enables production of an electric double layer capacitor and
a polarizable electrode exhibiting excellent charge/discharge
characteristics at high current density.
* * * * *